Triphenylmethyl tetrafluoroborate oxidation

In l,6,6aA4-trithiapentalenes or in l-oxa-6,6aA4-dithiapentalenes having a 3,4-propano bridge, triphenylmethyl tetrafluoroborate oxidizes the cyclohexane ring to a benzene ring with formation of a cation (7lAHC(l3)l6l,p. 198). 

The parent system is prepared by oxidation of tetrahydrothiopyran with triphenylmethane and perchloric acid (66HCA2046) in high yield, and the method is applicable to the preparation of virtually all simple alkyl or aryl derivatives. In more complex cases, especially where the strong acid might cause elimination or isomerization side reactions, triphenylmethyl tetrafluoroborate may be used with the thiin precursor (equation 113) (78CL723). 

The 1,3-dioxolane and 1,3-dioxane systems are stable to an extremely wide range of reaction procedures provided that an acidic medium is avoided. Deprotection is effected under a wide variety of mild acid conditions such as, for example, aqueous tartaric acid,134 perchloric acid,135 or moist silica gel either alone or in the presence of oxalic or sulphuric acids.136 The mild oxidative cleavage reaction with triphenylmethyl tetrafluoroborate in dichloromethane is... 

A further example of conversion of a thiopyran into a thiopyrylium salt is offered by compound 123, which has been oxidized to the corresponding thiopyrylium cation 79 (R = H, n = 2) by triphenylmethyl chloride, tro-pylium tetrafluoroborate, silver nitrate, and l,3,5,7-tetramethyl-2-phenyl-2-azoniaazulene (127) (74IZV1831). 

Oxidation by means of acetyl perchlorate or tetrafluoroborate of 1,2-dialkylcyclopropene carboxylic acids, themselves obtained by carbene addition to the appropriate alkyne, has afforded a method for obtaining dialkylcyclopropenium salts [4,5]. Under these conditions, 1,2-diphenylcyclopropene carboxylic acid gives only a very low yield of a diphenylcyclopropenium salt, the major product being 1,2,4,5-tetraphenylbenzeneCdurene) [22,23], This acid is converted into a l,2-diphenyl-3-triphenylmethylcyclopropenium salt when it is treated with triphenylmethyl perchlorate in methylene chloride and lithium carbonate [24]. 

The requisite tricarbonyl(ii -cyclohexadienylium)iron cation 2 is prepared from 1,3-cyclohexadiene and FefCOlj followed by treatment with triphenylmethyl carbocation tetrafluoroborate as shown. Subsequent oxidative cyclization of iron complex 3 with commercial manganese dioxide affords the iron-complexed dihydrocarbazol-3-one 4. 

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